ITFI20080155A1 - BROADBAND COMMUNICATION SYSTEM BETWEEN ONE OR MORE CONTROL CENTERS AND ONE OR MORE MOBILE UNITS - Google Patents

Description

BROADBAND COMMUNICATION SYSTEM BETWEEN ONE OR MORE CENTERS OF

CONTROL AND ONE OR MORE MOBILE UNITS

DESCRIPTION

The present invention relates to a broadband communication system between one or more Control Centers (CC) and one or more Mobile Units (MU) moving at high speed.

The communication between the rapidly moving element, typically a means of transport for people and / or goods, is supported by a wireless radio system equipped with roaming with HandOver (HO) between the network (on-board system) installed on the unit mobile and ground system network.

In this context, means of transport include, for example, cars, lorries, trains for suburban or underground railway lines, trams, trolley buses, ships / boats / rafts, trolleys for waste transport devices and the like.

Speaking of goods, we can also refer, for example, to raw materials, finished or semi-finished products, by-products, waste, etc.

Radio systems currently known to support communications in all environments where the speed of the vehicle in motion is a characteristic element, such as Universal Mobile Telecommunication System (UMTS), Global System for Mobile communications (GSM) or Terrestrial Trunked Radio (TETRA ), are limited in terms of bandwidth and supported services and cannot be used effectively when the extension of the bandwidth of the services to be provided (such as in the case of simultaneous video stream transmission from different cameras and live with high quality, without slowing down or freezing of images in the receiving station, high-speed data transmission in real time, video broadcasting in real time, voice communication in real time) is an essential requirement.

The aforementioned limitations, in terms of effectiveness and provision of the `` live '' attributes and real time to the services provided, currently affect the use of WLAN (Wireless Local Area Network) technologies defined by the IEEE 802.11 FHSS, IEEE 802.11a standards. , b, g and 802.16, in all cases where the speed of the moving vehicle is a characterizing element. It is clear, moreover, that, unlike what happens in a home or an office, where roaming rarely occurs and deferred communication is tolerated, supervision carried out through radio communication systems requires continuous communication in which roaming it must be carried out efficiently, respecting the constraints imposed by the type of services that must be ensured (live, data transmission, etc.), and often occurs at very high speeds.

Going into further detail, according to the known art, a network established to ensure communications between the subsystems installed on the mobile units and one or more control centers is called the Communication System (CS). The CS is an integrated, transparent Ethernet-IP network, which includes both wired and wireless networks. The CS is therefore a set of wired and wireless radio frequency network equipment, protected by a solid security system, and is based on commercial components, software and open standard protocols, interconnected and functionally integrated according to architectures and software specifically developed for make the required applications possible.

In practice, it includes a combination of switch / hub devices, equipped with suitable access interfaces (Ethernet or other) and radio, interconnected by a network of fiber optic cables, copper cables and radio links. The Ethernet switches / hubs are installed in technical rooms and have a dual purpose, that is to interconnect and aggregate the radio access points (Access Points - APs) to the network and constitute a high-speed Ethernet backbone. The interconnection of the APs is carried out by means of multi / single-modal fiber optic cabling and electro-optical converters, copper cables and radio links, to the network access switches. The high-speed backbone is obtained by connecting the Ethernet / IP switches together using single-mode fiber optic cabling and / or radio connection.

The APs are generally placed in fixed positions and act as an access interface between the wireless coverage area and the network hubs / switches. The access points for these applications are normally installed in environments subject to critical environmental conditions and are housed in containers that ensure protection according to the standard established for each specific environment (temperature, vibrations, wind, resistance, etc.).

The on-board mobile network is installed on the moving vehicle (car, plane, subway, train, etc.). Depending on the size of the vehicle in motion, the architecture of the mobile network can vary significantly, with the aim of obtaining maximum effectiveness from the Wi-Fi radio technology used.

The location of the APs must ensure uniform signal intensity in the area of interest. The distribution of the APs along the path of the MU must be established according to the roaming and connection thresholds previously established for the MU, which in turn is a function of the level of the interfering background noise.

The APs must provide a total coverage of the area with a minimum signal level above the measured background noise congruent with the coverage objectives and the minimum signal / noise ratios established to guarantee the predetermined project goals (min. And max. established bandwidth, min. and max. throughput, etc).

Once the background noise has been determined, it is possible to establish the minimum signal coverage necessary to ensure system throughput and to determine the positioning of the APs (see Figure 1).

One of the main optimization elements, in order to minimize the number of APs that guarantee coverage of the area in question, consists in limiting the overlap of the radio cells between the adjacent APs to the minimum necessary (figure 2).

To this end, it is desirable to reduce the pre-handover times (scan and search for a new AP with a better signal) and handover (disconnection from the AP to which the MU is connected and reconnection to the adjacent AP with a better signal. previously identified by the MU).

The concept of radio roaming therefore involves a series of associations and connections, disconnections and reconnections between the MU and the APs. During the roaming process, the MU is solely responsible for the association process with the APs.

A disconnection between the MU and the AP occurs when an existing connection is interrupted because the received signal level falls below the threshold established according to the criteria described above. A disconnection can be the initiative of the MU and / or the PA. Reassociation occurs when the MU re-associates to a new AP or to an AP it was previously associated with.

At any given moment, a MU can be associated with only one AP, ensuring that the MU only maintains a connection to the network. An AP, on the other hand, can be associated with several MUs at any given time.

The 802.11 standard outlines the functionality for roaming from the coverage area of one AP to the coverage area of another AP. The traditional roaming logic implemented in 802.11 devices is based on a selection process, where the premise for pairing to the next best AP is based on moving the MU towards a stronger signal while the existing signal is attenuating.

The handover procedure can therefore be divided, as anticipated, into two logical phases: identification and re-authentication, where the device performing the handover is the Wi-Fi radio unit installed in the on-board CU (Control Unit) of the MU.

The identification (or scan) can be expressed in the following terms. Due to the movement (for example) of the train along the tracks, the signal strength and the signal-to-noise ratio of the link deteriorate. A handover algorithm, implemented in the Wi-Fi Radio Card (RC) installed on the Mobile Unit, starts searching for the new AP by performing the active scan of the selected frequency range at the MAC (Media Access Control) level.

As for reauthentication, when the Wi-Fi card finds a new AP whose signal is above a predefined value, the connection to the new AP is allowed.

In complex networks, the process can be performed in coordination with a system controller that is part of the ground system network. The system controller knows the position of the MUs and CUs on the line. During the handover process, it manages the reassociation of the MUs from the old AP to the new one and re-routes the data sent (train-to-ground and ground-to-train) from the old AP to the next one to which the MU has reconnected (figure 3).

This process is performed all the way for all MUs connected to the network. The system controller is able to instantly manage the handover of all the MUs in the network ensuring the necessary parameters for maintaining the quality attributes (data flows, quality attributes for the transmissions that must be performed in real time, etc.) of all transmissions in progress.

In roaming mode, the moving MU selects the next best AP in a list of neighboring APs of which at least one has a signal level above the join threshold of the MU. This roaming logic guarantees the handover in topologies based on omnidirectional cells where the MU can move in any direction and where there are several APs (figure 4).

The conventional roaming logic implemented in the 802.11 specification gives no guarantee as to the time required for the MU to roam to the neighboring cell. Most of the overall handover duration is due to the scan phase.

This is one of the major limitations to support the aforementioned real-time services, particularly when the MU is moving at high speed. Depending on the speed of the MU, the connection can be interrupted for several seconds (see the self-explanatory diagrams in figures 5 and 6), giving rise to a degradation that in many cases can be completely unacceptable. In the diagram of figure 6 we note in particular in the time interval HO which represents precisely the duration of the handover, and the time interval Î "in which, due to this duration, there is disconnection and loss of data, in a much more the greater the speed of the vehicle is relevant.

The present invention aims to overcome this state of affairs by providing a more advanced broadband communication system between one or more control centers and one or more mobile units which ensures, among other things, thanks to a drastic speeding up of the scan phase, a transparent handover procedure, ie without connection drops, in an adaptable way according to the specific applications and circumstances of use.

This and other objects are achieved by the communication system according to the present invention, the essential characteristics of which are defined by the first of the appended claims.

The characteristics and advantages of the broadband communication system between one or more control centers and one or more mobile units will become clearer from the following description of one of its embodiments, provided by way of non-limiting example with reference to the attached drawings, in which:

Figure 1 and 2 are respective schematic representations of broadband communication systems according to the relevant known art of the present invention;

- figure 3 schematises at a macroscopic level a handover phase according to the known art;

- Figure 4 is a rough diagram representative of the concept of roaming; - figures 5 and figure 6 are respectively an illustrative diagram and a signal / time diagram of a known type of handover procedure;

- Figures 7 to 9 are diagrams and tables indicating the characteristics of the IEEE 802.11 standard;

Figures 10 and 11 are representative diagrams of a network architecture according to an embodiment of the invention;

- figure 12 is a simplified flow diagram of a handover procedure according to the invention;

- figure 13 is a signal / time diagram of a handover procedure according to the invention;

- figure 14 is a representative diagram of a system architecture according to the invention, in accordance with the ISO-OSI model;

- Figures 15a to 15e schematize at a macroscopic level the various phases of a handover procedure according to the invention;

- figure 16 is a block diagram showing the interactions between two control units arranged according to the invention in the same mobile unit;

- figure 17 is a graph of signal intensity in relation to the frequency of the wireless communication; And

- figure 18 is a diagram that exemplifies a distorting wireless signal path between a transmitting antenna and a receiving antenna.

With reference to said figures, a system according to the invention refers, although not specified below, to the general characteristics of the known systems mentioned above.

The reference transmission standard on which the system relies is that dictated by the IEEE 802.11 family of standards, well known to any expert in the field, which can be articulated according to the diagram in figure 7 and whose main characteristics and performances (in terms of width of bandwidth, modulation techniques and protection against interference) are summarized respectively by the self-explanatory tables in Figures 8 and 9.

In addition to the standards indicated in the diagrams and tables just mentioned, the system is also able to support the 802.11n standard, operating at 540 Mbit / s, currently awaiting final certification by the IEEE. The performance of the system will, moreover, be guaranteed not only by using the radio frequency bands provided for by the aforementioned standards, but in any other radio frequency band equipped with a bandwidth of the transmission channel compatible with that necessary for the services to be transmitted.

The frequency plan envisaged by the IEEE 802.11 standards divides the spectrum assigned in the 2.4 GHz range into 14 partially overlapping channels whose central frequencies are spaced 5 MHz from each other. All channels can be used simultaneously, but only 3 non-overlapping channels allow you to have the maximum bandwidth of 22 MHz, required by the standard, allowing the highest performance. Therefore, to achieve the highest system performance, it is necessary that the allocation of frequencies along the path of the moving unit allows for the maximum bandwidth.

The network architecture adopted is typically a communication system with interconnections based on copper / fiber optic cables and / or radio links, for example in a railway / underground environment, as shown in figures 10 and 11. Lâ € The invention can also be used with other types of networks such as Mesh networks and ad hoc networks. Mission critical applications, which require very high system reliability, operational continuity even in current crisis conditions and system recovery without interruption of the services offered, are preferably supported by a specially designed network architecture that can be traced back to the scheme of figure 11. Basically, the radio access points along the line are connected alternately to contiguous stations so that a breakdown or an accident in a station has no consequences on the service, which is ensured by the neighboring station.

Any degradation of performance consequent to the increased distance of the APs along the line, due to the non-functioning of the APs connected to the station out of service, is furthermore mitigated by the fact that according to an aspect of the present invention (as will become clearer below ), It is possible to exploit and indeed take advantage of the length of the vehicle with a multiplicity of control units distributed on it (for example one in the queue and one in the head of the train), so that this length can contribute to obviate the temporary increase in distance between APs.

Active and stand-by control centers are also provided, in order to ensure full functionality in the event of a fault or accident in one of them, the disaster recovery function being obtained thanks to the physical separation between the CC, the station equipment. and the APs.

The handover is managed by means of an algorithm created by means of a software through which the Wi-Fi radio device installed in the CU is piloted. This software is also referred to as a WLAN driver or simply WLAN.

Moving on to the main innovative aspects of the system according to the invention, it has been said of how the handover procedure can be divided into the two logical phases of scan and re-authentication, and of the responsibility of the scan phase, or rather of the relative delay, in determining the overall duration of the handover in known systems. On the contrary, the invention allows to minimize the scan phase, by introducing, in the procedure, the control and setting of a certain number of parameters, purposely managed according to a specific algorithm.

In fact, it is envisaged to control, in the handover algorithm according to the invention, a large number of parameters such as:

- time for which the APs are still considered valid by the application. At the end of the time a new scan is started and the list of valid APs is updated and these are used until a new scan;

- measurements of the received signal / noise ratio;

- threshold below which the bandwidth achievable with the transmission channel generates a roaming event;

- threshold below which the received signal level (Received Signal Strength Indication - RSSI) from the customer generates a roaming event;

- hysteresis threshold, ie the value by which the signal received (RSSI) from an AP must be greater than the AP to which the MU is currently connected to trigger a re-association event on the new AP;

- list of channels that must be scanned when a roaming event is started (the frequencies taken into consideration are only those foreseen by the frequency plan adopted along the path of the mobile unit);

- time that elapses between the scanning of one channel and the next;

- time that elapses between a comparison of the decision thresholds (transmission rate and RSSI) and the next one.

A handover procedure according to the invention can in practice be exemplified by the self-explanatory flow chart of figure 12. All the parameters mentioned are programmable and allow the system to adapt to any operating condition (speed of the mobile unit, topography of the route and of the tunnel, position of the antennas on the vehicle, etc.). This approach allows to minimize the scan phase (request and response), by specifically configuring the critical parameters for each application. It follows (figure 13) that a transparent handover is performed (HO time <20 msec) between neighboring APs in the direction of the moving vehicle (MU). This allows to remain below the maximum transmission interval of the data packets relating to those services, today mainly voice and video image transmission, for which this maximum interval must be ensured, in order not to have degraded the quality of the data received. to the ground.

In the event that the length of the mobile means requires, according to the criteria specified below, the use of additional CUs compared to the first, the handover is implemented with a specific application (â € œHandover Coordination Application -HCAâ €), installed at on board the MU (train, bus, car, truck, ship, etc.) and which is performed in a coordinated manner with the system controller on the ground.

According to the ISO-OSI reference model, the physical layer provides the hardware means for sending and receiving data on a radio bearer. The Data Link Layer is divided into two sub-levels: the MAC layer and the LLC (Logical Link Control) layer. The MAC sub-layer controls the MU's access to the network and authorizes the initiation of data transmission, while the LLC sub-layer manages frame synchronization, flow control and error checking. Figure 14 schematises the solution designed, in the case of two control units, in accordance with the ISO-OSI model.

In a further aspect of the invention, the on-board receiving-transmitting means provide a solution capable of optimizing the radio connection to the ground network with respect to the position of the AP access points. For this purpose, in fact, rather than a single CU operating with several antenna systems (a solution falling within the scope of the invention), on the MU are preferably installed, based on the length of the mobile unit, more CUs whose function is ̈ guarantee the optimal radio connection according to the definitions given in the previous paragraphs. The MU is radio-connected to the APs that are part of the ground network, through an on-board antenna system (each CU has its own antenna system), so that a single CU is always connected at a time. Selection / switching means are provided comprising a switch operating on the on-board LAN which connects the various CUs and is responsible for switching the on-board data flows.

The HCA is performed to ensure optimal management of all on-board CUs. It manages the physical levels (as defined above in relation to the ISOâ € "OSI models) and the signal received by the Wi-Fi radio unit of each of the CUs and decides which CU and antenna system must be used by the MU to communicate with the ground network.

According to a particular and advantageous innovative feature of the invention, the same MAC address is assigned to all the Wi-Fi radio units of the MU through the HCA running on the on-board CUs, in order to avoid complex and lengthy processing of the system controller. land operating at the second level of ISO-OSI. Thanks to this, the system controller on the ground "sees" all the Wi-Fi radio units installed on the MU as if they were one.

In the representations of figures 15a to 15e the handover execution is schematized in relation to the coordination functions between the on-board CUs performed by the HCA through the on-board LAN switch. In figure 15a there is a situation in which the Wi-Fi radio unit installed in a first CU (CU1) is making the radio connection and consequently the relative antenna (Ant1) is connected to an AP indicated with AP1. The HCA continuously checks the status of the Wi-Fi radio units installed in the on-board CUs and, by analyzing the carriers of the radiofrequency signals provided for in the frequency plan assigned to the MU, measures the critical parameters and evaluates the quality parameters that can be achieved in the event of a connection. . These parameters are compared with those detected by the other on-board CU (CU2) to decide which Wi-Fi radio unit to keep in scan mode, and which one to keep active for connection with the ground APs.

In figure 15b, the MU is moving between two AP positions (AP1 and AP2). When the HCA finds a signal carrier on CU2 that is better than the one on CU1, the internal switch is made between the control units, assigning the signal transmission to the CU2 Wi-Fi radio unit, and the handover between the units is commanded. two positions AP1 and AP2 adjacent. These two operations are performed simultaneously.

Subsequently (figure 15c) CU2 is responsible for the radio connection and through its antenna system (Ant2) it remains connected to AP2. In the situation of figure 15d there is still a connection between Ant2 and AP2 through the secondary radiation lobes of the antenna system of AP2. Finally (figure 15e) the HCA detects a better signal on CU1 than the one on CU2 and assigns the signal transmission, by switching only on the on-board system, back to C1.

In practice, the HCA running on the on-board CUs, assigning the same MAC address to all the Wi-Fi radio units of the MU, does not allow to detect that the transmitting radio unit has been changed and the radio link is maintained between the same AP2 and the secondary irradiation lobe of the Ant1 antenna system of the CU1. This allows to avoid the execution of the procedure foreseen by the standard to ensure the connection and the correct association of the data received in the cases in which the MAC variation is detected.

The sequence described above is repeated periodically during the movement of the MU on the path.

As regards the interactions between the control units, with reference to the exemplary diagram of figure 16, it should be noted in particular how CU1, CU2 and a CCTV server are connected on the same LAN and connected by one or more switches of the LAN network.

Data exchange is therefore made possible between all devices belonging to this local network. One of the on-board CUs (CU1 in this example) is assigned the function of network master and therefore acts as a default destination for user data. In this way a master-slave architecture is created, the determination of which CU acts as master being determined by a static (but modifiable) configuration parameter.

As explained, each of the applications residing on the CU (part of the HCA) checks the status of its own Wi-Fi radio system, the quality of the connection and the results of the scan, then communicates, through the HCA, with the same application residing on the other CU. The HCA, generally residing on the CU Master (CU1) decides which Wi-Fi radio unit must be in scan mode, and which one must be activated for connection to the ground network. This communication takes place via the on-board LAN switch interfaces.

In addition, the HCA performs the function of checking the functionality of the equipment and, in the event of failures or malfunctions, automatically redirects the data through the other operating CUs.

User data (e.g. CCTV video streams) are always routed to the default Master CU (CU1 in the example). If, at a certain time, this default CU is connected to the AP on the ground, it will send data directly through its Wi-Fi radio unit and antenna system. If, on the other hand, in the same instant another CU (for example CU2) is responsible for the vehicle-to-ground radio link, the Master CU routes the data to the other CU connected to the AP.

In case of non-optimal operation of a CU, a procedure may be activated which allows the transmission of the data flow to be maintained, possibly in a reduced mode. If necessary, the data to be transmitted are redirected to the CU which has the best quality parameters at that moment, possibly adapting the data flow transmission speed to the available band as a function of the quality parameters measured on the signal received by the CU itself.

The Wi-Fi radio unit (sometimes also called "WLAN driver" or simply WLAN, meaning the software that drives it, installed in it) is equipped with a buffer of adequate size in order to avoid losing packets when both procedures handover and internal switch are being implemented. The application manages the buffer data by synchronizing the storage when the application is started.

Different types of traffic can obviously be present on the system: CCTV stream to the CC, video transmission files from the CC, data transmission, VOIP, etc. traffic priority levels (e.g. traffic related to passenger safety, VOIP, etc.). IP addressing for the onboard system will preferably be configured so that each MU has a different IP subnet, and therefore the CU acts as a router from inside to outside the MU.

Going back, and going into further detail, on the HCA, the handover control application, it can for example be a Windows CE console application that communicates with the control application of the CU to decide which CU should be active. Then configure the radio unit according to the activity or inactivity of the local CU.

As mentioned, in each MU it is necessary to have a master CU and at least one slave CU. The master CU is the one that decides which CU should be active, while the slave CUs simply communicate their connection status to the master and execute its commands.

The HCA reads its configuration settings from the registry keys. The main elements of this register, and their possible settings, always to be considered purely by way of example for a possible embodiment, are provided below.

"Master CU IP address" - Required setting on slave CUs, you need to specify the IP address of the master CU assigned to the Ethernet port.

"WLAN Device Name" - In the case of systems with a single WLAN interface, the default value is usually sufficient to ensure operation, and no further specification is required.

"Signal to Noise Ratio Threshold" - Sets the dB difference of the Signal to Noise Ratio (SNR - Signal to Noise Ratio) that must be exceeded to start the handover. This threshold is used to avoid continuous switching from one CU to another if their signal-to-noise ratio is approximately equal.

"Handover Delay" - Sets the time interval in ms in which the signal-to-noise ratio of a CU must be at least better than the dB value of the SNR threshold than the currently active CU for handover to start. Since SNR measurements may vary slightly, this avoids handover activation due to a single wrong SNR measurement.

"Command Line Options" - Specifies a list of predefined command line options to switch to the WLAN HCA executable. This is useful when starting the HCA automatically at system startup, as there is no other way to specify command line options in this case.

The Wiâ € “Fi radio unit driven by its own WLAN driver, i.e. the specific software installed in it, designed according to the criteria set out above to allow handover in times compatible with the transmission of the services mentioned above (fast handover) , then use the following registry key for your specific settings.

"Network address" - Defines the manual assignment of a MAC address to be used instead of the one stored in the EEPROM of the WLAN card. This is necessary since both CUs of a train must have the same MAC address. However, CUs from different trains must have different MAC addresses. The suggested strategy for choosing the MAC address is to use a locally administered MAC address (bit 1 of the first byte must be set) and derive the last bytes of the address from the IP address assigned to the train (since it must also be unique for the train). None of the MAC addresses stored in a CU EEPROM need to be configured here, as the HCA uses these when the CU is idle, which would result in collisions between active and inactive CUs.

"Inhibit Association" - This key makes a setting that prevents automatic association with an access point during system startup. It is necessary because the CUs of a train use the same MAC address, so if they both automatically bind to an AP at startup, the AP would see two clients with the same MAC address. Upon system startup, the HCA takes control of this setting and makes sure there is only one CU associated at any given time.

"Disable Radio" - Same setting as the "Inhibit Pairing" setting just mentioned, completely disables the radio during startup until the HCA takes over. In this way, probe requests are not sent until a unique MAC address is configured.

"AP Network Name" - Sets the network name of the APs the CU should connect to. Obviously, it must be the same for all CUs to allow for a correct handover.

"Antenna Switch" - Key that disables the automatic antenna switch / diversion, recommended if only one antenna is connected, as it avoids using the open antenna connector completely. Depending on where the antenna is connected, this value may require different settings for the other connector.

"Disable Background Scan" - Enable / disable background scan. The concept of handover is not based on background scan, so it is advisable to disable it to avoid unwanted traffic interruption while performing this operation.

"Channel Scan" - Limitation of the channels scanned to those used for 802.11g in order to make the scan faster. Alternatively, through a parameter "Channel list" it is possible to manually specify the channels to be scanned to make the scan even faster. It is recommended that you specify either the "Channel Scan" or "Channel List" parameters, but not both at the same time.

In addition to the specific settings for the Wi-Fi radio units mentioned above, the standard IP parameters for the Wi-Fi radio interface must also be configured in a sub-key (mandatory settings "IP address", "Subnet", "Gateway default "," Enable DHCP ")

It is important to note that DHCP must be disabled for handover to work, so IP addresses must be statically assigned. All CUs in a MU must be configured to use the same IP address in their Wi-Fi radio interface to avoid address changes during handover.

The HCA makes sure that only one CU uses its IP address at any given time to avoid duplicate IP addresses. The configured gateway IP address should be the IP address of the central router located in the backbone network.

The IP addressing scheme for the handover concept ultimately has only two simple requirements: each MU needs its own unique IP subnet for the on-board equipment and at least one additional IP subnet is required to contain all Wi-Fi radio equipment (both the APs and the radio interfaces of the CUs). In this way, the CU can route information between these two networks.

Obviously, the size of the IP subnets prefixes must be chosen so that the onboard subnets provide a sufficient number of addresses for all on-board equipment of a single vehicle and the WLAN subnet must be able to provide one host address per vehicle, as well as one host for the central router. Of course, the onboard and WLAN subnets should use different prefix sizes.

It is also important to note that the central router needs one inbound route per vehicle with associated IP address assigned to the CUs of this particular MU.

For normal operation, the HCA will generally be configured to start automatically when the system is started. Only one instance of the HCA will be running in a CU at any given time and the HCA will ensure automatic protection against multiple calls.

Returning to the general characteristics and performance of the system, the design of a uniform radio coverage is essential in order to ensure the provision of a wireless communication service without interruptions. The strategy used to maintain the radio communication service without solution of continuity, during the rapid movement of the mobile unit, is carried out by means of the appropriate management of the following parameters: location of the APs and intensity of the signal; selection of antennas and their orientation on the AP and MU; network architecture for MUs and operating strategy for mobile network units; threshold settings of MUs and APs; configuration and diversity of antennas with respect to coverage parameters; Handover algorithm (roaming) and Handover strategy; appropriate verification and management of interference to the radio signal.

Any unbalanced combination of these elements will produce abnormal system behavior within the affected area, such as incorrect AP - MU associations, excessive retransmissions, excessive packet loss and / or unpredictable system behavior that compromises the ability to deliver services whose basic requirement are â € œliveâ € and real-time attributes, such as high-quality video streaming, VOIP services and high-speed data transmission in real time. In addition to what has already been considered previously, other aspects of the system according to the invention therefore deserve consideration.

Interference due to background noise is one of the main causes that determine the intensity of the necessary radio signal, since the operation of the system as required by the 802.11 standards is based on suitable values of the effective signal / noise ratio. The minimum signal intensity threshold for an MU-AP association should be between 12dBm and 18dBm above the identified interference / background noise level (Figure 17).

Multipath interference occurs when a wireless signal has more than one path between a receiver and a transmitter. This means (figure 18) that there may be more than one path that the RF signal travels from a transmitting (TX) antenna to a receiving (RX) antenna. These multiple signals combine in the RX antenna and receiver causing further signal distortion.

The paths through which the signal propagates have different lengths, so the propagation time of the signal is different between one path and another: as a result, multiple signals arrive at the receiver with slightly different intervals. Another type of diversity to consider when moving the HM occurs when the quality of the wireless radio signal changes from instant to instant as the HM moves closer to or away from the associated AP signal. The antenna gives the radio system three fundamental properties: gain, directivity and polarization. The gain is the measure of the increase in the energy radiated by the antenna in a predetermined direction, the directivity is determined by the shape of the radiation pattern in space while the polarization is determined by the planes of the antennas according to which the signal is propagated. Each antenna is characterized by these parameters which determine the achievable coverage. Increasing the gain of the antenna also increases the distance covered, but only in a certain direction.

To overcome the phenomenon known as multipath distortion or multipath fading, it is preferable to use different antenna systems, with two antennas, for which the Wi-Fi radio device installed on the CU of the MU alternately receives the signal from one antenna or the other. , listening until a valid radio signal is received. The system can be compared to a switch that selects antennas individually but never both simultaneously. Upon receipt of the initial synchronization of a valid packet, the radio evaluates the packet synchronization signal on one antenna and then switches to the other. The radio then selects the best antenna and uses only that for the remainder of that packet. When transmitting, the system selects the same antenna used for the last communication. If a packet is lost, switch to the other antenna and try to send the packet again.

The adoption of this scheme is particularly advantageous if adapted, according to the invention, to use on a vehicle such as a train, since the two antennas can be placed at a short distance from each other, exploiting the external geometry of the MU, differentiating and optimizing the characteristics of the antennas (strongly directive or omnidirectional), according to the position on the vehicle, to achieve greater efficiency of radio signal transceiving between mobile and fixed networks. Typically, for example on a train, a distinctly unidirectional antenna will be advantageously placed in the front position, while an antenna with greater omnidirectional characteristics will be placed on the roof of the train.

Finally, it is important to reiterate how the roaming / connection thresholds of the MU must be set in order to maintain the required signal / noise ratio as a function of the interference / background noise present in the entire spectrum. A roaming threshold set below the proper SNR signal-to-noise ratio can cause prolonged MU - AP association to the point where the MU loses signal. This condition results in intermittent disconnections with connection loss, re-scans and re-associations, resulting in excessive retransmissions and / or lost packets.

A connection threshold set below an adequate SNR involves the association MU - AP in conditions of weak signal / noise ratio with consequent decrease of the actual data flow transmitted in the unit of time and excessive retransmissions and / or packets lost. As for the APs, these must offer full area coverage with a continuous minimum signal level suitably above the measured background noise.

Thanks to the invention which is the subject of this patent application, a system with a broadband transmission channel and a wide range of supported services is therefore offered while maintaining the â € œliveâ € œ characteristics of the same, up to the full use of the width of available bandwidth, while the means of transport moves at maximum speed.

The radio communication bandwidth made possible by the system allows it to support simultaneous high quality live video streaming from multiple cameras, real-time high-speed data transmission, real-time video transmission and real-time voice communication between media rapidly moving and one or more centers on the ground.

Bandwidth can be managed effectively so that it is shared simultaneously between the broadcast services while maintaining and ensuring the real-time transmission requirement between the fixed and mobile networks.

Variations and / or modifications may be made to the broadband communication system between one or more control centers and one or more mobile units without thereby departing from the scope of the invention as defined by the attached claims.

Claims (15)

CLAIMS 1. A telecommunication system comprising at least one control center, at least one mobile unit along a route comprising a plurality of stations, said at least one mobile unit and said at least one control center being able to manage a bidirectional data / signal communication to broadband with roaming between said mobile unit and a plurality of wireless repeaters, integrated in the network with said at least one control center and distributed along said path, said at least one mobile unit comprising each on-board transceiver-receiving means adapted to communicate with said repeaters wireless, characterized in that it provides a control application capable of continuously scanning the quality of said broadband communication, based on the consideration of at least some of the following parameters: time for which the repeaters are considered valid; received signal / noise ratio; threshold below which the bandwidth achievable with the transmission channel generates a roaming event; threshold below which the received signal level generates a roaming event; hysteresis threshold of the signal received by said repeaters; predefined and programmable list of a plurality of channels to be scanned when a roaming event is initiated; time that elapses between the scanning of one channel and the next; time that passes between a comparison of said decision thresholds and the next. 2. System according to claim 1, wherein said on-board receiving-transmitting means comprise one or more control units, according to the length of said unitable, each communicating independently with at least one of said repeaters, and selection means, suitable for to continuously scan the quality of said broadband communication on said control units according to said control application, and to decide by comparison which of said control units is enabled for communication, excluding the other or others. 3. System according to claim 2, wherein said selection means comprise switching means operating on a LAN connecting said control unit. 4. System according to claim 2 or 3, comprising at least one pair of control units arranged, with their own antenna means, respectively at the head and tail of said mobile unit. 5. System according to claim 4, wherein each control unit is associated with a different antenna system comprising two antennas, whereby the signal is received alternately by one antenna or the other, the control unit remaining listening until a valid radio signal is received. 6. System according to claim 5, in which said two antennas are arranged in a close proximity comprising an antenna arranged in frontal position on the mobile unit, and an antenna on the roof of the mobile unit, whose transmission characteristics of marked directionality or omnidirectionality are set on the basis of the design parameters, including mainly the propagation characteristics, of the specific application. 7. System according to any one of claims 2 to 6, wherein said control application assigns all the control units of said mobile unit the same MAC address, not allowing to detect that the transmitting radio unit has been changed following a roaming event, thus avoiding the execution of the standard procedures envisaged to ensure the connection and correct association of the exchanged data in the cases in which the change in MAC is detected. 8. System according to any one of the preceding claims, in which the roaming event is driven following a detection of the transmission rate lower than a threshold value, or from a detection of the received signal / noise ratio lower than the threshold value , or by a detection of elapsed time exceeding a predetermined maximum time to deem the repeaters of a given list valid. System according to claim 8, wherein the roaming event is followed by an update of the list of valid repeaters. 10. System according to claim 8 or 9, in which following said roaming event there is re-association with a new repeater only if for one of the repeaters in said list a value of the signal / noise ratio greater than the value of the signal / noise ratio of the previously connected repeater added to a predetermined hysteresis value. 11. System according to any one of claims 2 to 10, in which one of the on-board control units is assigned the function of master of the network and therefore acts as a predefined destination of the user data, whereby a ™ master-slave architecture, the determination of which control unit acts as the master being determined by a static but modifiable configuration parameter. System according to any one of claims 2 to 11, wherein said control application automatically directs data / signal towards the operating control units, in case of malfunction of one or more control units, maintaining the transmission of the data flow, possibly in a reduced way. System according to any one of the preceding claims, in which said control unit comprises buffer memories of dimensions suitable for avoiding the loss of data packets when the scanning and switching phase is in progress. System according to any one of the preceding claims, in which in said network said repeaters are alternatively connected to neighboring stations so that a malfunction of a station does not prevent a regular service from being ensured by the adjacent station, the network further comprising control centers active and in stand-by, physically separated from said repeaters and station hubs. System according to any one of the preceding claims, in which, given the adoption of a frequency plan which divides the spectrum assigned into a multiplicity of partially overlapping channels, three non-overlapping channels are used which allow the maximum bandwidth to be available.